Why Does Sexual Dimorphism Exist in Mud Crabs? Figure 3 - When we look closer at the last two spines (spine 8 and spine 9) of the carapace of mud crabs as marked in yellow triangle, it is obvious that the spine 9 of orange mud crab would not exceed the length of spine 8, but the spine 9 of purple and green mud crabs are always longer than their spine 8.However, purple and green mud crabs both have the widest carapace at spine 9 instead of spine 8 ( Figure 3). In the orange mud crab, the carapace width at spine 8 is similar or larger than the carapace width at spine 9. Scientists number these spines from 1 to 9, to more easily refer to them. In Figure 2, you can see that mud crabs have spines along the top edges of their carapaces. We also discovered an interesting feature of orange mud crabs. Chelipeds have three main parts: the propodus, the merus, and the dactyl ( Figure 2), and males commonly have larger dactyl length, propodus length, and propodus width. In addition, males also have larger chelipeds. Males normally have larger bodies than females, as measured by carapace width at spine 9, internal carapace width, carapace length, and carapace at spine 8. In all three species, the abdomens of females are larger than those of males. We found that, in general, sexual dimorphism patterns are similar among orange, green and purple mud crabs. Figure 2 - For this study, we measured various body parts including carapace length (CL), internal carapace width (ICW), carapace width at spine 9 (CW), carapace width at spine 8 (8CW), the right cheliped’s propodus length (PL), the right cheliped’s propodus width (PW), the right cheliped’s merus length (ML), the right cheliped’s dactyl length (DL) and the right cheliped’s propodus depth (PD).The measurements we took are shown in Figure 2. We measured a few representative characteristics for each category. To make the comparison easier, we divided the body parts into three main categories: carapace, right cheliped, and abdomen. To study this, we measured several body parts of mature males and females from the three mud crab species. We asked if the three species show the same sexual dimorphism patterns. We studied three different species-orange, green, and purple mud crabs-that live together in the same habitat. There are differences besides the abdomens, too! We were interested to know exactly which characteristics show obvious differences between male and female mud crabs. Males have narrow, triangular abdomens, whereas females have broader abdomens ( Figure 1). You can easily tell male and female crabs apart just by examining their abdomen shapes. Sexual Dimorphism in CrabsĬrabs, like most animals, are sexually dimorphic too! We studied mud crabs, which are commonly found in estuaries and mangrove forests in the Asia Pacific region. Sexual dimorphism exists in almost all animals, although in some species the male and female characteristics might not be as noticeable as they are in humans. Other characteristics, such as larger antlers or horns, may be used to show dominance over other males in species like deer and antelopes. Some may exist to attract the opposite sex, such as the large, colorful tails of peacocks. These differences between males and females of the same species are called sexual dimorphism. For example, most adult men have thick facial hair and deeper voices, while women lack facial hair and have softer voices, among other features. In humans, it is generally easy to distinguish men and women by their visible characteristics. Telling Males and Females Apart: Sexual Dimorphism Sexual dimorphism in crabs might be linked to how they reproduce and where they live. Unlike the green and purple species, orange mud crabs are wider at spine 8 than spine 9. All mud crabs have spines along the top edge of their shells, and scientists have numbered these spines to identify them. Orange mud crabs also have a unique body shape variation compared to green and purple mud crabs. Males generally have larger body and claw sizes, whereas females have larger abdomens. We found that three mud crab species (orange, green, and purple mud crabs) showed similar patterns of sexual dimorphism. Sexual dimorphism may help species to survive in their environments. These variations between sexes are known as sexual dimorphism. Male and female mud crabs may look alike at first glance, but clear differences in physical characteristics exist.